1. Field of the Invention
This invention relates to a mother glass (i.e., homogeneous glass before ion exchange) composition suited for producing graded index lenses having a refractive index gradient in its cross section. More particularly, it relates to a glass composition for graded index lenses, which is excellent in drawability and ion-exchanging properties.
2. Description of the Related Art
A graded index lens is a rod lens having a refractive index distribution in its cross section varying from the center axis toward the periphery. It has many advantages such that it has an image forming action even with flatness on both sides and can easily be processed into a microfine lens. Therefore, it has recently enjoyed wide application to an optical system for copiers, facsimiles, LED array printers, liquid crystal shutter printers, etc.
A graded index lens having such wide applicability is usually produced by an ion-exchange process, in which a glass body containing a first cation capable of constituting a modified oxide is immersed in a high-temperature molten salt containing a second cation capable of constituting a modified oxide to exchange the first cation in the glass body for the second cation in the molten salt thereby forming a refractive index distribution changing from the center of the glass body toward the periphery.
For example, it is known that a Cs-containing glass body undergoes ion exchange in a molten potassium nitrate to provide a graded index lens having excellent chromatic aberration (see JP-B-51-21594). The angular aperture of this lens is as small as about 10xc2x0 at the most. JP-B-59-41934 discloses a process for obtaining a graded index lens having a greater angular aperture, in which Li-containing glass body is subjected to ion exchange in molten sodium nitrate.
However, experimentation following the working examples of JP-B-59-41934 revealed that the Abbe""s number, which is a measure (index) of dispersion of light, of the composition containing lead oxide is as small as less than 40. The recent broadening of application of graded index lenses has demanded reduction of dispersion, namely, an increase of the Abbe""s number. On the other hand, the composition containing no lead oxide has an extremely high devitrification temperature and is not fit for large-scale production. Further, a glass composition having no lead oxide tends to develop cracks on ion exchanging.
An object of the present invention is to provide a mother glass composition for graded index lenses which has a small dispersion and suitability to large-scale production.
In order to solve the above-described problems of conventional techniques, the present inventors have conducted extensive investigations and, as a result, have found that incorporation of specific amounts of specific components which are selected from oxides of the metal elements having an atomic number of 30 (Y) to 73 (Ta) and which hardly cause coloration into PbO-free SiO2-TiO2-Li2O-Na2O-based glass provides a glass composition which has a desirable refractive index, hardly undergoes devitrification, and hardly develops cracks on ion exchanging and is suitable as a mother glass composition for a graded index lens.
The mother glass composition for graded index lenses according to the invention comprises, as glass components,
45 to 65 mol % of SiO2,
2 to 12 mol % of TiO2,
2 to 22 mol % of MgO,
1 to 16 mol % of BaO,
8 to 10.5 mol % of Li2O,
5 to 12 mol % of Na2O, and
1 to 15 mol %, in total, of ZnO, Y2O3, ZrO2, Nb2O5, In2O3, La2O3, and Ta2O5, provided that the content of ZnO is 0 to 5 mol% and the content of each of Y2O3, ZrO2, Nb2O5, In2O3, La2O3, and Ta2O5 is 0 to 7 mol %.
The content of BaO is preferably equal to or less than that of MgO, in order to prevent devitrification. Furthermore, it is preferred that the content of ZnO is 1 to 5 mol % with the total content of ZnO, ZrO2, Y2O3, and La2O3 being 3 to 10 mol %, in order to suppress crack development during ion exchange. Moreover, the total content of Y2O3 and La2O3 is preferably 1 to 9 mol %.